Data communication system with prioritized periodic and aperiodic messages

Abstract

A slotted CSMA contention technique and arrangement is disclosed for use in a radio data communications systems having a central base station/controller and a plurality of remote data terminals operating over inbound and outbound channels. The channels are divided into major frames, which are further divided into minor frames, which in turn are divided into microslots. An arrangement is devised whereby channel access may be prioritized as between periodic and aperiodic messages by advantageously utilizing the occurrence of these microslots so as to give priority to the former while completely eliminating collisions therebetween and, at the same time, optimally controlling the efficiency of the system as a whole.

Accordingly, what is claimed is: 
     
       1. In a data communications system having inbound and outbound communication channels on which a central station and a plurality of remote terminal units operate, a slotted CSMA contention arrangement for selectively controlling the operation of such remote terminal units, comprising in combination: said inbound channel being divided into a plurality of standardized microslots of a given duration and wherein said outbound channel is adapted to convey inbound channel busy status information; and   means for prioritizing access to the inbound channel by the remote units when transmitting periodic and aperiodic messages thereon,   said remote units being permitted to transmit periodic messages at the start of the first microslot after the inbound channel is sensed not busy, and to transmit aperiodic messages only after waiting an additional one microslot to determine the inbound channel remains not busy.   
     
     
       2. A data communication system in accordance with claim 1 wherein a given number of said microslots form a respective minor frame, a given number of which form a respective major frame. 
     
     
       3. A data communication system in accordance with claim 2 wherein said means for prioritizing channel access includes means for granting sole use of a selected minor frame to a requesting remote terminal for the duration of an on-going periodic message. 
     
     
       4. A data communication system in accordance with claim 3 wherein said request for channel access by a remote terminal to transmit a periodic message is by an aperiodic data message over inbound channel and which is granted by a similar aperiodic message for the outbound channel. 
     
     
       5. A data communication system in accordance with claim 2 wherein said channel access prioritization means further includes means for automatically indicating said inbound channel is busy two microslots prior to the assigned minor frame so as to prevent unwanted acquisition by a radio data terminal transmitting an aperiodic message. 
     
     
       6. A data communication system in accordance with claim 1 which further includes means for synchronizing the remote terminal to the start of an inbound microslot and which further includes means for aligning frame synchronization of the outbound messages with inbound microslots and requiring said remote terminal to synchronize to these sequences and maintain internal timing referenced to said sequences. 
     
     
       7. A data communication system in accordance with claim 2 wherein said inbound channel busy status includes means for providing busy bits interspersed with data words on the outbound channel and wherein the probability of collisions on the inbound channel due to bit errors is reduced by means which requires the remote terminals to sample m busy bits and sense that the inbound channel is not busy only when all such sampled busy bits so indicate. 
     
     
       8. A data communication system in accordance with claim 2 wherein maximum access delay to the inbound channel is optimized for periodic messages by means limiting the maximum length of aperiodic messages to one minor frame or less. 
     
     
       9. In a data communications system having inbound and outbound communication channels on which a central station and a plurality of remote terminal units operate, a method for selectively controlling the system remote terminal units by a slotted CSMA technique, comprising the step of: dividing the inbound channel into a plurality of standardized microslots of a given duration and utilizing the outbound channel to convey inbound channel busy status information; and   prioritizing access to the inbound channel by the remote units when transmitting periodic and aperiodic messages thereon,   permitting remote units to transmit periodic messages at the start of the first microslot after the inbound channel is sensed not busy, but aperiodic messages only after waiting an additional one microslot to determine the inbound channel remains not busy.   
     
     
       10. A method for controlling a data communication system in accordance with claim 9 which includes the further step of utilizing a given number of said microslots to form a respective minor frame and a given number of such minor frames to form a respective major frame. 
     
     
       11. A method for controlling a data communication system in accordance with claim 10 wherein prioritizing channel access includes the further step of granting sole use of a selected minor frame to a requesting remote terminal for the duration of an on-going periodic message. 
     
     
       12. A method for controlling a data communication system in accordance with claim 11 wherein the request for channel access by a remote terminal to transmit a periodic message is made by an aperiodic data message over inbound channel and which is granted by a similar aperiodic message for the outbound channel. 
     
     
       13. A method for controlling a data communication system in accordance with claim 10 wherein the step of prioritizing channel access includes the steps of automatically indicating said inbound channel is busy two microslots prior to the assigned minor frame so as to prevent unwanted acquisition by a radio data terminal transmitting an aperiodic message. 
     
     
       14. A method for controlling a data communication system in accordance with claim 9 which further includes the step of synchronizing the remote terminal to the start of an inbound microslot and aligning frame synchronization of outbound messages with inbound microslots and requiring said remote terminal to synchronize to these sequences and maintain internal timing reference to said sequences. 
     
     
       15. A method for controlling a data communication system in accordance with claim 10 wherein busy bits are interspersed with data words on the outbound channel to indicate inbound channel busy station and wherein the probability of collisions on the inbound channel due to bit errors may be reduced by requiring the remote terminals to sample m busy bits and sense that the inbound channel is not busy only when all such sampled busy bits so indicate. 
     
     
       16. A method for controlling a data communication system in accordance with claim 10 wherein maximum access delay to the inbound channel is optimized for periodic messages by the further step of limiting the maximum length of aperiodic messages to one minor frame or less.